An experimental animal model in which the course of immunodeficiency virus infection parallels the pathogenesis of the human disease is critical for the study of the pathogenesis of human AIDS. Simian immunodeficiency virus (SIV) infection of macaques satisfies this criterion and is therefore a relevant model. SIV induces an immunodeficiency syndrome in infected macaques that is remarkably similar in pathogenesis to human AIDS. An important use of this animal model system is the detailed study of pathogenesis and viral determinants of disease since many studies of this type are not feasible in humans. Such investigations should allow us to determine how primate lentiviruses destroy the immune system of their hosts, and facilitate the development of more rational therapeutic antiviral strategies. The purpose of this project is to investigate host and viral factors involved in variable disease progression in SIV-infected macaques. To investigate the role of host factors, we employed well-defined molecularly cloned viruses (SIVsmE543-3) to reduce the complexity of the inoculated virus as a contributing factor. Susceptibility to SIV infection of primary PBMC in vitro varies significantly between individual macaques. Interestingly, the susceptibility phenotype correlates with the extent of in vivo viral replication following inoculation of these animals. To investigate this phenomenon as a potential factor affecting disease progression, we characterized a cohort of six rhesus macaques for susceptibility phenotype and then inoculated them with SIVsmE543. Susceptibility of PBMC in vitro was highly predictive of subsequent viremia. The two highly susceptible macaques exhibited high level of plasma viremia; one failed to develop a transient antibody and CTL response and was euthanized due to AIDS by 16 weeks. Two macaques of the intermediate phenotype had significant viremia and two macaques with a low degree of susceptibility had very low viremia (negative by SIV p27 antigen during primary infection). In order to evaluate the magnitude of the immune defect in such animals, macaques were immunized with tetanus toxoid and Hepatitis A (Haverix). While two SIV-naive macaques and three SIV-infected macaques responded appropriately to these antigens, the rapid progressor failed to develop antibody to either antigen. These data suggest a global immune defect in rapid progressor macaques. To characterize viral factors involved in pathogenesis, studies are ongoing to molecularly and biologically characterize the virus in macaques that progress rapidly to AIDS in the absence of an SIV-specific immune response. Sequence analysis of env genes cloned from three SIVsm-infected animals exhibited common substitutions in the env gene. These substitutions were unusual in that they involved residues that were generally conserved and that were known to affect binding of env to CD4 or coreceptor. Ghost cell assays revealed a broadening of coreceptor usage of isolates derived from these animals. Studies are ongoing to evaluate the pathogenesis of one such isolate in a naive cohort of rhesus macaques in parallel with the parental virus, SIVsmE543-3. We also have performed two related studies of the pathogenic determinants of the acutely lethal SIVsmPBj6.6 molecular clone. One study evaluated which of five specific substitutions in the minimally pathogenic PBj6.9 clone were responsible for its decreased virulence as compared to PBj6.6. The D119G substitution in envelope of SIVsmPBj6.9 was associated with a marked reduction in infectivity of this virus relative to SIVsmPBj6.6. An associated processing defect in gp160 of SIVsmPBj6.9 and chimeras expressing the D119G substitution suggests that a reduction in virion envelope incorporation is the mechanistic basis for reduced virion infectivity. In vivo studies revealed that substitution of the 6.9 amino acid into PBj6.6 (D-119) abrogated the pathogenicity of this previously pathogenic virus. Introduction of the PBj6.9 G-119, however, did not confer full virulence to the parental PBj6.9, implicating one or all of the other four substitutions in the virulence of SIVsmPBj6.6. We also evaluated the role of the tyrosine at position 17 of SIVsmPBj Nef in the acute pathogenesis of this virus by introducing this unique residue into AIDS-inducing clones, SIVsmE543-3 and SIVagm9063-2. The introduction of Tyr-17 conferred the ability to replicate in resting macaque PBMC and altered the pathogenesis of the AIDS-inducing viruses. Thus these animals exhibited some of the acute symptoms and pathology of macaques inoculated with SIVsmPBj although the disease was less severe. However, the Tyr-17 did not alter pathogenesis by increasing viral replication in the host since the kinetics and extent of viral replication were similar in macaques inoculated with either Y+ or Y- clones.